Baobabs are one of the most graceful trees on Earth, thanks in part to their unusual appearance. Their cartoonishly thick trunks are conspicuously large in relation to their tiny crowns, earning them the nickname “upside-down trees”. They can also live for thousands of years, contributing to their prominence in cultural traditions and artwork.
However, for all the stories told about baobabs, the story of their origin has remained a mystery.
Scientists have debated for years how baobabs wind up in the places where they grow. Eight species are found worldwide, and their distribution, like the trees themselves, is unusual: One species occurs across much of mainland Africa, while six occur in Madagascar. The latter is far away, in northwestern Australia.
Most researchers have assumed that the trees originated on the African continent. But findings published Wednesday in the journal Nature tell a different story. Baobab instead of most likely first evolved in Madagascar, where they differentiated into different species. Two then embarked on long-distance sea voyages to distant continents.
“Madagascar is this wonderful natural laboratory,” said Tao Wan, a botanist at the Chinese Academy of Sciences’ Wuhan Botanical Garden and author of the new study. He added, “In the case of baobabs, some very particular geographic history on the island contributed to the diversity of the species.”
Dr Wan and his colleagues sequenced the genomes of all eight baobab species and then used this data to understand how the trees evolved. They also investigated ecological factors that affected the distribution of baobabs around Madagascar.
Their results show that the common ancestor of baobabs probably appeared in Madagascar about 21 million years ago. Competition with other plants and factors such as altitude, temperature, rainfall and volcanic activity caused new baobab species to appear in Madagascar, as did sea level fluctuations during various ice ages.
Baobabs also probably evolved a mutualistic relationship with lemurs that served as pollinators. Other relatively large animals, including fruit-eating bats and bush babies in Africa, began to visit the nocturnal flowers of baobabs for nectar. “One of the evolutionary innovations of baobabs was the exploitation of large sugar-eating animals,” said Andrew Leitch, a plant geneticist at Queen Mary University of London and an author of the study. “That’s an unusual thing for a plant.”
At some point, probably around 12 million years ago, two species of Madagascar baobab found their way to mainland Africa and Australia, where they evolved into the only trees that grow there today. Most likely, many baobab seeds hitched a ride as the plant was carried by the Indian Ocean gyre, a current that circulates counter-clockwise between Australia, South Asia and the east coast of Africa – exemplifying its “fascinating and unusual long-distance dispersal patterns” kind of. said Dr. Leitch.
“Baobabs are amazing trees, so I was excited to see this paper,” said Pamela Soltis, a botanist at the University of Florida who was not involved in the work. He added that the research offered new perspectives on the evolution of the baobab.
In addition to filling in missing pieces of the evolutionary puzzle, the authors’ findings also raise conservation concerns. Two of Madagascar’s species have alarmingly low genetic diversity, indicating that they may not have the resilience needed to adapt to climate change. A third species is also at risk of extinction due to interbreeding with a more widespread cousin.
These three species are already on the International Union for Conservation of Nature’s Red List as endangered. The new genetic findings suggest their conservation status should be reassessed and possibly upgraded to even higher levels of threat, said Ilia Leitch, a plant geneticist at the Royal Botanic Gardens in Kew and an author of the paper.
All six Madagascar baobab species are also affected by an ongoing wave of disappearances which has been happening in Madagascar for the past 2,500 years and which researchers say is mainly due to human activity. Several species of giant lemurs—some of which grew to gorilla-sized proportions, and all likely served as key seed dispersers for baobabs—were hunted to extinction about 1,000 years ago. Almost all of the forest understory that surrounded Madagascar’s baobabs has also been lost to recent development.
While species naturally come and go in evolutionary history, “this process is exacerbated by human intervention,” said Dr. Ilia Leitch.
